1. Field of the Invention
Antenna performance and size cause a large impact on the development of wireless devices. Moreover, development of wireless devices greatly depends on improvement of antenna characteristics and size. Designing a traditional antenna that provides fine typical parameters like bandwidth, efficiency and gain within a limited antenna volume is extremely hard. Antenna design is even more critical in devices using the ultra wideband frequency range (“UWB”) because communication in UWB systems uses very high data rates and low power densities.
2. Description of the Related Art
Printed antennas are extensively used in various fields due to their many advantages such as their low profile, light weight, easy fabrication, and low cost.
Antennas are grouped generally into resonant-type antennas and non-resonant-type antennas. When a resonant-type antenna acts at its resonant frequency, almost all power of the resonant antenna can be radiated from the antenna. However, when the receiving or transmitting frequency is different from the resonant frequency, the received or transmitted power cannot be delivered or radiated efficiently. Because of this, the resonant antenna is used by connecting many antennas of different resonating frequencies to each other to cover a wide frequency range. On the other hand, the non-resonant antenna can cover a wide frequency range, but realizing high antenna efficiency in a wide frequency range is very difficult. Additionally, antennas having good frequency characteristics in a wide frequency range and high efficiency are usually large. Therefore, normal antennas are not adaptable to wireless devices using the UWB frequency range because the devices have to be small, light and low cost.
FIG. 16 shows an example of a prior art micro-strip antenna having a rectangular slot. A metal layer 111 is layered on an insulation substrate 110. A rectangular slot 112 is formed in the metal layer 111. The metal layer 111 is connected to a transmission line 114 via a pin 113 inserted through the substrate 110. Transmission power is fed from a transmission circuit (not shown) connected to the transmission line 114 to the metal layer 111. When receiving an electric wave, the electric wave is received by the metal layer 111, and the signal is transmitted to a receiving circuit (not shown) connected to the transmission line 114 (see, for example, the microstrip antenna described in non-patent document 8 discussed below).
The following are references to related art. Prior art microstrip antennas are described in non-patent documents [1–6]. Prior art slot antennas are described in non-patent documents [7–8].    [1] G. Kumar and K. C. Gupta, “Directly coupled multi resonator wide-band microstrip antenna,” IEEE Trans. Antennas Propagation, vol. 33, pp. 588–593, June 1985.    [2] K. L. Wong and W. S. Hsu, “Broadband triangular microstrip antenna with U-shaped slot,” Elec. Lett., vol. 33, pp. 2085–2087, 1997.    [3] F. Yang, X. X. Zhang, X. Ye, Y. Rahmat-Samii, “Wide-band E-shaped patch antenna for wireless communication,” IEEE Trans. Antennas Propagation, vol. 49, pp. 1094–1100, July 2001.    [4] A. K. Shackelford, K. F. Lee, and K. M. Luk, “Design of small-size wide-bandwidth microstrip-patch antenna,” IEEE Antennas Propagation Magz., vol. 45, pp. 75–83, February 2003.    [5] J. Y. Chiou, J. Y. Sze, K. L. Wong, “A broad-band CPW-fed strip-loaded square slot antenna,” IEEE Trans. Antennas Propagation, vol. 51, pp. 719–721, April 2003.    [6] N. Herscovici, Z. Sipus, and D. Bonefacic, “Circularly polarized single-fed wide-band microstrip patch,” IEEE Trans. Antennas Propagation, vol. 51, pp. 1277–1280, June 2003.    [7] H. Iwasaki, “A circularly polarized small-size microstrip antenna with a cross slot,” IEEE Trans. Antennas Propagation, vol. 44, pp. 1399–1401, October 1996.    [8] W. S. Chen, “Single-feed dual-frequency rectangular microstrip antenna with square slot,” Electron. Lett., Vol. 34, pp. 231–232, February 1998.
Prior art microstrip antennas are disadvantageous because of their narrow-band frequency range. For an antenna to be suitable for UWB wireless devices, the antenna must be small, light, have wide bandwidth, and have low manufacturing costs. Traditional microstrip antennas, with or without slots, cannot not achieve these conditions.